Bedub - jI've been organising events for the Jeepclub for a decade all over Europe, so couple of comments:
Just pick a date and go with it. The more you post alternatives the more endless haggling of people who can and can't attend.
Avoid bank holidays and school hoidays as many people struggle to do those weekends (not me these days

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Make it expressly clear that this is not an organised event and get a disclaimer and registration signed by everyone who attends. Sorry to be a pain, but to ///M's point on the likes of PLI. If someone is stupid, hurts themselves or others, damages property, etc. then you can personally be sued for unlimited amounts.
Have a set of 'rules' that are known, as in the heat of the day people do some strange things.
Have a simple safety briefing.
This is not Silverstone and the same sort of precautions have protected a small group playing in the mud on hillsides for years.
Bit of care and it should be a great day with a group of friends just enjoying their cars
